Connecting elements directionally so that information flows only one way is a requirement in signal processing. Directional elements route information from a source to a destination, while simultaneously preventing interference from signals passing through the chain in reverse. Canonically, separating an input from an output is performed by the circulator. A circulator is a component with a number of ports (typically three or four) that can operate in a sequence such that an input signal arriving at one port is output from the next port in the sequence.
A second important directional device is the two-port amplifier, which is used to increase signal power levels that are otherwise too weak to be successfully processed by subsequent elements. The directionality of an amplifier is specified by the reverse gain, i.e., the gain experienced by a signal traversing the amplifier in reverse, which is usually much less than unity for a directional amplifier.